Field of the Invention
The present invention relates to a turbocharger which includes a bypass passage constituting a bypass between an upstream and a downstream of a turbine wheel (turbine impeller).
Description of the Related Art
A turbocharger rotates a turbine wheel by using energy of an exhaust gas, and rotates a compressor wheel (compressor impeller) by the rotation of the turbine wheel through a shaft, thereby increasing a boost pressure of air which flows into an engine. The exhaust gas having rotated the turbine wheel is discharged to the outside from a discharge passage provided in a turbine housing. However, the boost pressure may be increased too high if the energy of the exhaust gas is excessive. To avoid this, there is a widespread configuration, such as a turbocharger described in Patent Document 1, which is provided with a bypass passage constituting a bypass between an upstream and a downstream of a turbine wheel, and with a waste gate valve located inside a discharge passage where an outlet end of the bypass passage is formed. When an actuator opens the waste gate valve, part of the exhaust gas flows into the downstream of the turbine wheel by bypassing the turbine wheel, and an increase in the boost pressure is thereby suppressed.
In the recent structure, due to the necessity to control a concentration of air pollutants in an exhaust gas, an exhaust emission control system is connected to a downstream side of a discharge passage of a turbocharger and removes the air pollutants. However, a catalyst in the exhaust emission control system for removing the air pollutants is not heated to an appropriate temperature at the start of an engine (at a cold start). Hence, the exhaust emission control system can hardly exert a sufficient emission control function. Accordingly, a turbocharger described in Japanese Utility Model Registration Application Publication No. Sho 60-131633 (Patent Document 1) includes a second bypass passage which connects a supply pipe for supplying an exhaust gas to the turbocharger and a discharge pipe for discharging the exhaust gas from the turbocharger. In addition, an integrated valve including two valve discs respectively provided on opposite sides is connected to a stem, and is turned about an axis of the stem by a second actuator provided separately from an actuator to operate a waste gate valve. The integrated valve is arranged in a positional relationship where, by turning the axis of the stem, the integrated valve can bring one of the valve discs into contact with a seat surface located at an outlet end of the discharge pipe or can bring the other valve disc into contact with the other seat surface located at an outlet end of the bypass passage. When the integrated valve blocks the discharge pipe at the cold start, the exhaust gas is guided to a catalyst by way of the second bypass passage. Thus, without consuming energy for rotating a turbine wheel, the high-temperature exhaust gas is directly guided to the catalyst, and hastens a rise in temperature of the catalyst.